The stm-split package

Transactional MVars, Vars and Channels with distinguished input and output side.
When threads communicate via a TMVar, a TVar or a TChan
there are often clearly defined roles,
which thread is the sender and which one is receiver.
We provide wrappers around the standard concurrency communication channels
that make the distinction clear and type safe.

For example, if a function has a parameter of type TChan.In
then it is sure that it will only write to that channel.
Additionally if the compiler warns about an unused TChan.Out
that was created by TChan.new
then you know that the receiver part of your communication is missing.

See also package concurrent-split for non-transactional communication.
This package follows the same idea as chan-split but is strictly Haskell 98.